The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device that drives a TFT (i.e., Thin Film Transistor) liquid crystal display panel by an alternation driving method.
As a conventional art that has taken into consideration the attainment voltage of a common voltage, JP-A-8-76083 has disclosed a liquid crystal display device where a positive or negative precharge voltage is added to a positive or negative driving voltage needed for the liquid crystal display. Also, JP-A-9-21995 has disclosed a liquid crystal display device where a differentiation signal generated with a predetermined time constant is superimposed on a common driving signal. Also, JP-A-10-253942 has disclosed the following liquid crystal display device: In a pixel where there has occurred a delay in the attainment voltage of the common voltage, a timing at which the TFT is switched off is set within a preparation time-period during which the output resistance of a source driving circuit has become a high resistance. This setting effectively decreases the load on a common voltage circuit at the time of immediately before the TFT is switched off, thereby intentionally causing an overshoot to occur in the common voltage at an instant when the output resistance of the source driving circuit has become the high resistance.
As a conventional art that has taken into consideration the alternation of a gate-off voltage, JP-A-2000-28992 has disclosed the following liquid crystal display device: The Low potential is varied in a state of being synchronized with a higher potential and a lower potential of the common potential Vcom, and, concerning a potential difference between the low potential and the common potential, the potential difference in the higher potential of the common potential is made larger than the potential difference in the lower potential of the common potential. Otherwise, the low potential is varied in synchronism with the higher and lower potentials of the common potential Vcom, and the potential difference in the higher potential of the common potential is made equal to the potential difference in the lower potential of the common potential.
In JP-A-8-76083, JP-A-9-21995, and JP-A-10-253942, no consideration has been given to the degradation in picture-quality referred to as xe2x80x9ctransverse smearxe2x80x9d. Namely, the final attainment voltage of the common voltage inside the liquid crystal panel is varied, depending on a load constant of the liquid crystal panel and a distortion in the common voltage due to the displayed contents. As a result, the voltage effective-value is varied for each display region (e.g., a region of an intermediate-luminance background alone, and background regions of the right and the left to a region where a rectangle on the white display is displayed). This gives rise to the picture-quality degradation referred to as the transverse smear such that the luminance differs for each display region.
In JP-A-2000-28992 either, no consideration has been given to the picture-quality degradation referred to as the transverse smear. Namely, in JP-A-2000-28992, the gate-off voltage is synchronized with the common voltage. As a result, the flowing in-and-out of current occurs in a cross capacitor and a parasitic capacitor, depending on the displayed contents. This slows the convergency of a liquid crystal panel""s input unit onto the potential level of a drain voltage, thereby decreasing for each display region the voltage effective-value applied to the liquid crystal panel. This gives rise to the picture-quality degradation referred to as the transverse smear.
It is an object of the present invention to provide a liquid crystal display device allowing the transverse smear to be suppressed and the picture-quality to be enhanced.
In the present invention, the common voltage outputted from the liquid crystal panel is fedback to a power-supply circuit for generating the common voltage to be applied to the liquid crystal panel. This makes it possible to improve the convergency of the common voltage inside the liquid crystal panel, thereby allowing the transverse smear to be suppressed and the picture-quality to be enhanced.
In the present invention, the gate-off voltage for switching off a gate in a switching element within the liquid crystal panel is made high-impedance. This makes it possible to improve the convergency of the drain voltage inside the liquid crystal panel, thereby allowing the transverse smear to be suppressed and the picture-quality to be enhanced.